The present invention relates to communication over a telephone line between a host computer and a remote computer and, more specifically, to a circuit for providing a "busy signal" on the telephone line when the host computer is resetting.
A computer connected to a telephone line via a modem may function as a host, receiving a telephone call initiated by a remote computer and establishing a remote computing session between the two computers. A host computer may operate an electronic bulletin board or other service in such a manner.
When the host modem is in an "auto-answer" mode, it responds to the incoming call by placing the telephone line in an "off-hook" state, transmitting a carrier tone, and waiting for a response from the remote modem. If the host modem does not receive a response within about 15 to 30 seconds, it returns the telephone line to the "on-hook" state, i.e., it hangs up. The "off-hook" state exists when a load is present across the TIP and RING signals of the telephone line.
The communications software that the host computer executes may reset or "boot" the host computer from time to time to ensure that the system is returned to a known state. Such software may, for example, reset the host computer each time a remote computing session is concluded.
A problem arises when a remote computer attempts to call the host computer before the host computer is ready to execute its communications software. In such a scenario, the host modem may answer the call, transmit a carrier tone and, after receiving a response form the remote modem, establish communications with the remote modem before the host computer has finished executing its boot routine. Thus, the host computer is unable to respond to the remote computer because the host computer is unaware of the presence of the call. The remote computer must hang up the call and try again.
A similar problem arises when an error in the host computer hardware or software prevents the host computer from completing its boot routine. If the host modem answers an incoming call after such a failure has occurred, the remote computer must hang up the call.
It would be desirable to prevent a remote modem from establishing communications with a host modem before the host computer is ready. This problem is clearly felt in the art and is solved by the present invention in the manner described below.